Silicon is often brought up in science fiction as being very similar to carbon, just below it on the periodic table. The silicon-based "organic" molecules are more tightly bound and thus would find a higher temperature to be appropriate.

Being too hot for liquid water, what would it use as a solvent? That is, what would the Horta drink?

Hal Clement wrote a novel where the aliens found Earth to be extremely cold, so much so that their base on Mercury's (thought at the time to be tidally locked) day side was further heated another hundred degrees with reflected sunlight. They breathed sulfur (an analog of oxygen) as a gas. I don't recall if he went into the chemistry in more detail.

Given a toolkit of a few kinds of atoms with different numbers of binding sites, the heavier ones in the same period might substitute for what we are familiar with, to a first approximation.

So once you switch Silicon for Carbon, what other changes might be useful to make a workable toolkit-for-life? What solvents are available/possible in different temperature and pressure regimes?

$\begingroup$The main deficiency of my answer is the fact that it relies on standard pressure for the temperature ranges. I couldn't find a quick and easy discussion of how different pressures change the values.$\endgroup$
– Jim2BMar 31 '16 at 20:21

$\begingroup$Hmm... you are not talking about silicone life forms... ?$\endgroup$
– BurkiApr 1 '16 at 14:25

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$\begingroup$Silicone is a polymer that uses silicon (rather than carbon). It's not a life form. (However, silicone is implanted in some life forms.)$\endgroup$
– JDługoszApr 1 '16 at 17:04

$\begingroup$I think I remember the Hal Clement story. The aliens were having a problem with addiction to 'tofacco' because at their atmospheric temperature is was a gas.$\endgroup$
– Howard MillerMay 27 '16 at 16:43

$\begingroup$Yes, the cigarettes were cut up into doses and stored cryogenicly in a block of frozen sulpher (air). When released in the room, it probably combusted with sulpher as an analogy with how oxygen works in our world. It certainly vaporized, but the details were not explained. In reality, I think eartly "organic molecules" of any kind would completely break up at that temperature.$\endgroup$
– JDługoszMay 28 '16 at 0:59

At the same temperature and pressure extremes you find Earthly life, silicon is inferior to carbon because it cannot form long chains with itself. At temperature and pressure extremes where carbon-based life could not survive (e.g. Venusian), silicon-based polymers would remain stable. (Source)

Silicon would need to alternate with another element such as oxygen, nitrogen, or boron in order to form long chains, but otherwise it would have much the same potential for polymers as carbon. We ourselves use silicon-based polymers for lubricant and plastics. They would likely use all the same elements that we do, though in vastly different arrangements.

Silicon-based life wouldn't be able to respire oxygen the same way we do for several reasons:

Silicon dioxide is a solid, so whenever they exhale they would
produce a fine dust (or a brick?!) instead of a gas. This is vastly
inefficient compared to exhaling gas.

Since they likely would not use water as their solvent of choice,
then free oxygen could not exist in their atmosphere to begin with.
The reason why our atmosphere is rich in oxygen is because plants,
cyanobacteria and other photosynthetic autotrophs extract the oxygen
from water during photosynthesis and exhale it as a waste product. (Source)

Halogens such as chlorine and fluorine may produce gaseous compounds with silicon, such as the silicon tetrafluoride naturally dumped into our own atmosphere by volcanoes. However, halogens are heavier and more reactive than oxygen (and flame-retardant) and thus silicon life would need to compensate. However, their sun would have to release much stronger UV radiation to power their photosynthesis to extract halogens. (Source)

Since we're assuming an environmental temperature above the boiling point of water and the decomposing point of glucose so that silicon is preferred to carbon, sulfuric acid (or another sulfur compound if above the boiling point of sulfuric acid) is the most obvious potential solvent. (Source)

In any case, there would not be a one-to-one correspondence between the chemical equations for biological processes between silicon and carbon lifeforms since they couldn't utilize their equivalent solvents and reagents the same way. Synthesis could react, for example, SiF4 with HF or HCl (gaseous or dissolved in solution) to produce sugar analogues and free F or Cl to power respiration. Respiration would likewise react F or Cl with the sugar analogues to produce energy and exhale gaseous SiF4 and HF or HCl as waste. Already their biochemistry is noticeably different because they don't use their solvent as their electron donor and exhale two types of gaseous wastes.

$\begingroup$Thanks for the post. I feel overall that it's more "some notes" rather than an Answer. But, I upvoted due to the inclusion of source citations!$\endgroup$
– JDługoszMay 26 '16 at 23:59

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$\begingroup$I am sorry I could not be of more help. Designing a completely different biochemistry from scratch would be intensely time consuming (believe me, it took many hours of work just to get the right synthesis and respiration). May I recommend the Speculative Evolution Wiki?$\endgroup$
– AnonymousMay 27 '16 at 12:39

$\begingroup$I've never heard of that before.$\endgroup$
– JDługoszMay 28 '16 at 0:50

$\begingroup$The life might be breathe oxygen, but not for the purpose of oxidizing silicon (since SiO2 is a solid at every reasonable temperature under consideration). I'd expect that for such a system of biology to exist, there needs to be a way to converting SiO2 into some other compound of silicon that could act as a structural element, analogous to (although very dissimilar in actual composition) either an amino acid or simple sugar (from which Terran life is constructed).$\endgroup$
– EvilSnackMay 28 '16 at 21:35

The term I’ve seen used to describe the necessary solvents for silicon-based life is cryosolvent. In Planets and Life: The Emerging Science of Astrobiology, by Sullivan & Ross (relevant excerpt here), the authors cite the work of Bains (2004) on various alternate solvents to form the basis for various alien biochemistries. Ethane, methane, and liquid dinitrogen are all considered as possibilities, with liquid dinitrogen showing the most promise for silicon-based life.

At low temperatures, silicon-silicon chains can exist, containing up to 30 silicon atoms and mimicking the structure of certain carbon polymers1. As noted here, these chains aren’t as stable as carbon chains, and less stable in many liquids. However, this becomes less of a problem at low temperatures. Furthermore, as Bains’ website says, silicon may be the only option for liquid dinitrogen.

Almost paradoxically, silicon-based life has advantages over carbon at high temperatures, as the Center for Astrophysics discusses! Silicon-oxygen and silicon-aluminum bonds can withstand temperatures hundreds of kelvin above temperatures on Earth. However, the lack of a good solvent that is compatible with silicon at these temperatures is a stumbling block. Sulfuric acid has been considered as a solvent, but not, as far as I know, with silicon.

As mentioned here, silicon photosynthesis is also possible. However, be careful before drawing a direct analogy between silicon-based life and carbon-based life in terms of respiration. As shown here, SiO2 produced from respiration (the analogue to CO2) would be solid, which would make it very difficult for an organism to breathe.

Bains writes on his website that you could create a system analogous to the Krebs Cycle using ethylene, acetylene, and water, which would generate CO2 and methane, but he doesn't discuss if and how fast this could happen at low temperatures. You still have the problem of instability of silicon-silicon chains when faced with water.

1 As this report concluded, “hybrid” chains of the form SinC2nH2n+1On+1 are also possible. They would look like this:

I'm not aware of the temperature regimes in which they could remain stable, though.

$\begingroup$So the solvent is available at low temperature, but will the chemestry work at low temp. or be , well, frozen, without energy at the level of the bonds? Any metabolic process would boil the cytoplasm.$\endgroup$
– JDługoszJun 1 '16 at 6:51

$\begingroup$@JDługosz I don't know; I can't read the cited paper. It does seem that at higher temperatures, though, things could work, if you could find another solvent (see my edit).$\endgroup$
– HDE 226868♦Jun 2 '16 at 20:20

$\begingroup$Venusian (e.g. 90 atm, 462C) would be a good environment. Sulfuric acid boils at 770C under 90 atm. For comparison, water boils at 279C under 90 atm.$\endgroup$
– AnonymousJun 7 '16 at 14:41

I'm posting this answer to consolidate some points raised in the original answers by various people, and expand/clarify points that could have been better-written answers.

Some Useful References

A very interesting book, available for free on the web now, is Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization by Robert A. Freitas Jr. This was written in 1975–1979, so it's a bit dated. But, §8.2.3, Alternatives to Carbon, is pretty much what this question is about.

An oft-used reference for Worldbuilding is the Atomic Rockets web site. The page Building Blocks covers alternatives to life as we know it.

The web page Alternative Forms of Life on David Darling's Encyclopedia of Science is similar. It's an index of pages in the same body.

Hot or Cold ?

First of all, let's clarify what “silicon based” bio-molecules might entail. It's well-known that a difference between Si and C is that Si has higher binding strength, and things like silicon grease are produced for high temperature use. So, we conclude that Si analogues of Carbon-bearing molecules would need (and withstand) higher temperatures.

Well, that's not quite right. Our grease and silly putty and caulk contain Si, but are not direct analogues to hydrocarbon molecules. They contain alternating silicon and oxygen as a replacement for carbon in the main chain, and furthermore the side chains are conventional carbon-containing chains.

(( Figure 8.4 in Freitas was made on a typewriter. I wonder if some modern renderings could be done and posted here? Imagine illustration of Polydimethylsiloxane, Phenylsilicone, etc.))

So, high temperature molecules would not be simply silicon based, and still needs carbon. Freitas points out that the increased temperature tolerance is modest and probably not worth it.

First, many silicones tend to disassemble into ring molecules at temperatures of roughly 300–350 °C. (Similar behavior is observed in most complex carbon compounds, but at somewhat lower temperatures.) It would be difficult for silicones to remain stable in much hotter climes, and it is unclear whether this slight thermal advantage is enough to enable Si to out-compete C in a high temperature regime.

On the flip side, pure silicon chains, analogous to carbon chains, are possible at cryogenic temperatures. Furthermore, liquid nitrogen is a suitable solvent. So rather than the Horta, we have Nadreck the Palainian. But, this raises the issue that the energy levels for metabolism would be too high and you would blow everything apart instead of doing the combining and splitting of other molecules that being alive is all about.

So, What Other Elements?

It turns out that complex molecules need carbon as well as silicon. Some useful high-temperature polymers use Si, 0, and H in abundance, and also incorporate some more exotic elements: Germanium, Tin, and Lead for example is shown in Freitas figure 8.4.

If trying to avoid carbon, you can often use half-and-half Boron and Nitrogen instead. Again, Freitas shows this in figure 8.6.

I expected more alternative atoms, like Sulfur instead of Oxygen, to go with the high-temperate idea. But that seems not to be the case, and the molecules we are familiar with use common organic side chains when producing products to work in the 250–300°C range.

However, as Freitas relates, reporting in the mid 1970s about discoveries made shortly before, H. R. Allcock remarked “… it now seems likely that almost any set of required properties can be designed into the polymer by a judicious choice of side groups.”

So we may have all kinds of elements, many more than we have in familiar biochemistry. Instead of an odd metal atom or other oddball as part of a critical catalyst, we might have an abundant number of some other element atoms used as part of the polymer repeating unit, and a smorgasbord of elements in the side chains of useful molecules, as this is how to fine-tune their properties.

It seems to me that this would make it harder to evolve a simple general-purpose toolkit of common parts. Common parts might be based on larger units than familiar life, with several kind of complex molecules all using a large (from our point of view) number of different elements, rather than just the C-H-O-N.

Summary

High-temperature life that uses Silicon will use all the same elements we do, and all the other elements it has available as well. Rather than getting varied and carefully tuned properties from different arrangements of a few kinds of atom, this alternate life would need to use many more kinds of elements to achieve the careful tailoring of properties.

The most basic reusable building blocks would be rather large, compared to ours, since it takes a complex molecule with side chains to make it do exactly what is needed. And evolving a new novel distinct molecule for every need, to make a more basic reusable set they need to be general purpose tools that can be reused as-is.

We have a lack of universal solvent. The idea of cytoplasm will be very different, with different ways to deliver, distribute, react these building blocks. That is not elaborated here but ought to be a new question.